Hi
You already have some commercial contracts and it about exploring what techniques you used to obtain these. Meaning the method you may have used to obtained a Domestic contract may have been different to the one you used to secure your Commercial contract.
Start by deciding on what Commercial industry you are trying to target and then set about a driven campaign. This could be in the form of flyers, letters, emails etc., whatever method you use ( and there are many). The difficulty is the message or content of your introductory letter or flyer that needs to be persuasive. This has to be right and strike a cord with your intended customer, therefore by doing research and finding out what your customer needs is a start. After that don't wait for the campaign to work for you, you drive it, i.e. you contact the client, visit the client what ever form you take to get a result or foot in the door.
There is nothing wrong with a 'Scattered Gun' or 'Spam' approach to marketing but yes it is time consuming, can be costly if no budget is set and can have a low success rate if not driven and monitored. This is the part of the business that requires a lot of effort and is so often overlooked and can leave you feeling like your efforts are wasted.
REMEBER - It's only wasted if you are WAITING FOR A RESPONSE rather than DRIVING THE RESPONSE!!!!!!!!!
I hope this helps